We have described previously the use of naphthalocyanines as IR-absorbing dyes. Naphthalocyanines, and particularly gallium naphthalocyanines, have low absorption in the visible range and intense absorption in the near-IR region (750-810 nm). Accordingly, naphthalocyanines are attractive compounds for use in invisible inks. The Applicant's U.S. Pat. Nos. 7,148,345 and 7,122,076 (the contents of which are herein incorporated by reference) describe in detail the use of naphthalocyanine dyes in the formulation of inks suitable for printing invisible (or barely visible) coded data onto a substrate. Detection of the coded data by an optical sensing device can be used to invoke a response in a remote computer system. Hence, the substrate is interactive by virtue of the coded data printed thereon.
The Applicant's netpage and Hyperlabel® systems, which makes use of interactive substrates printed with coded data, are described extensively in the cross-referenced patents and patent applications above (the contents of which are herein incorporated by reference).
In the anticipation of widespread adoption of netpage and Hyperlabel® technologies, there exists a considerable need to develop efficient syntheses of dyes suitable for use in inks for printing coded data. As foreshadowed above, naphthalocyanines and especially gallium naphthalocyanines are excellent candidates for such dyes and, as a consequence, there is a growing need to synthesize naphthalocyanines efficiently and in high yield on a large scale.
Naphthalocyanines are challenging compounds to synthesize on a large-scale. In U.S. Pat. Nos. 7,148,345 and 7,122,076, we described an efficient route to naphthalocyanines via macrocyclization of naphthalene-2,3-dicarbonitrile. Scheme 1 shows a route to the sulfonated gallium naphthalocyanine 1 from naphthalene-2,3-dicarbonitrile 2, as described in U.S. Pat. No. 7,148,345.

However, a problem with this route to naphthalocyanines is that the starting material 2 is expensive. Furthermore, naphthalene-2,3-dicarbonitrile 2 is prepared from two expensive building blocks: tetrabromo-o-xylene 3 and fumaronitrile 4, neither of which can be readily prepared in multi-kilogram quantities.

Accordingly, if naphthalocyanines are to be used in large-scale applications, there is a need to improve on existing syntheses.